If you use a membership plugin on your WordPress site, users who sign up for a membership can be given access to your Discourse forum. Without adding some custom code, the only way to do this is to add the members to your Discourse forum manually. With some custom code, members can be automatically added to your Discourse forum when they sign up on WordPress.
The difficulty in automatically adding members to a forum is that there are quite a few WordPress membership plugins, and the all work a little differently.
I've created a plugin that integrates the wp-discourse plugin with the WishList Member plugin. It still needs some testing, so I haven't released it publicly, but if you'd like to try it out, get in touch with me.
There is someone who has requested a plugin to integrate wp-discourse with the Memberships Pro plugin. That plugin has a free version, so I'm interested in seeing if I can get it to work.